1. Field of the Invention
This invention relates to motor drive systems and in particular to reversible motor drives in which the travel of the drive mechanism is limited by mechanical blockage.
2. Description of Related Art
In many different types of small devices it is necessary to limit the travel of a particular mechanical movement and to then reverse the direction of travel. It is usual to interrupt the travel at the desired point by use of some device for sensing when the moving structure has reached some predetermined position. For example, a mechanical switch, a photodetector or other sensing device may be used to control the drive system. In many low-power drives, however, where cost and space are important considerations, it is advantageous to merely stop the motor by mechanical means and then to remove or reverse the current to the motor by responding to the lack of motion or the typical increase in the current drawn by the electric motor. Such a system may utilize a slip clutch in series with the motor, as described in the above-mentioned application Ser. No. 07/727,647, now U.S. Pat. No. 5,184,260. In most instances, however, the use of mechanical stop means has been avoided by the use of more expensive sensor mechanisms. One reason for using such sensors is because of the tendency of such drive systems to "freeze" at the end of the mechanical stroke, particularly in drives having a substantial speed reduction, into a condition where the typical low-power inexpensive motor does not generate sufficient starting torque to initiate the drive in the opposite direction.